So far, friction materials comprising asbestos as a reinforcing substrate and a phenolic resin as a binding resin (binder) have been used for a number of applications, because of their excellent cost performance. However, asbestos was found to be carcinogenic, and asbestos-containing friction materials naturally discharge asbestos, which has posed the problem of environmental pollution.
Therefore, it has been studied to use, in place of asbestos, any suitable combinations of materials more expensive than asbestos, such as glass fibers, rock wool, steel fibers, brass fibers, aramide fibers, hard carbon, and calcined coke. For example, a friction material comprising steel fibers and wholly aromatic polyamide fibers (Japanese Laid-Open Patent Publication No. Sho 56-88433), and a friction material comprising glass fibers (Japanese Laid-Open Patent Publication No. Sho 51-87549) are available.
Initially, friction materials containing glass fibers or steel fibers as a reinforcing substrate were developed. For example, Japanese Laid-Open Patent Publication No. Sho 48-16972 proposes a method of producing a clutch facing using glass fibers. Glass fibers are linear and suitable as a roving for a clutch facing. However, they are unsatisfactory in terms of opposite-surface wearing properties and friction coefficient. Compared with asbestos, moreover, they are poor in preform retaining properties and may easily collapse. Thus, they are problematical in productivity, and their use is becoming rare. Inorganic fibers including glass fibers, if they are very thin fibers of several microns or less in diameter, may be carcinogenic like asbestos. Hence, it is difficult for such very thin fibers to maintain productivity comparable to that of asbestos.
A semimetallic disk brake pad containing steel fibers as a reinforcing substrate has been found to present a noticeable squeal of the brake, rusting, high heat conductivity, and other drawbacks.
Fibers or a fibrillated pulp of an aramide resin, one of wholly aromatic polyamides, on the other hand, are or is a reinforcing substrate with excellent wear resistance and free from the above-described defects, such as opposite-surface attacking properties and a brake squeal. The fibrillated pulp also has the advantage that it has excellent properties of retaining the shape of a preform in a widely used method for the production of a friction material which comprises producing the preform under pressure, laying the preform on a base plate having an adhesive layer formed thereon, and molding the assembly with heating under pressure for integration. The fibrillated pulp, however, has high water absorption, and is poor in affinity for a phenolic resin or a bismaleimide triazine resin. Upon rapid heating, it may blister due to steam, thereby causing the friction material to peel off. Owing to these problems, its amount usable is limited, thus resulting in insufficient strength. In producing the friction material, drying is an indispensable step for attaining a predetermined proportion of the fibrillated pulp. Moreover, the aramide fibers have the disadvantage that when they are cut, they tend to be fluffed on the cut surfaces.